The traditional construction of a solid-state photo-sensor pixel 200, illustrated in FIG. 1, consists of a semiconductor volume S in which an incident stream of photons In creates a proportional number of photo-charge pairs, a charge-collection mechanism (either diffusion or drift in an electric field caused by a photogate contacted with at least two contacts PG0, PG1), a charge-storage device D (either a conductor-oxide-semiconductor capacitance or a diffusion capacitance), a sensing or source-follower transistor T1, a reset switch T2, a pixel-select switch T3 and an electronic circuit for buffering or amplification of the collected photo-charge signal (realized, e.g., as a source follower, consisting of the source-follower transistor T1 and a current source that provides a bias current Ibias). This is explained in detail in P. Seitz, “Solid-state Image Sensing”, in “Handbook of Computer Vision and Applications”, eds. B. Jähne, H. Haussecker and P. Geissler, pp. 165-222, Academic Press, New York, 1999.
The total external quantum efficiency of the detection of visible and near infrared (NIR) irradiation with silicon easily surpasses 50%. Current restrictions in the sensitivity of semiconductor photo sensors are rather due to noise in the electronic photo-charge detection circuits. Three relevant noise sources can be identified as follows:                (i) reset noise, also called kTC noise, originating from the temperature dependent resistor (Johnson) noise in the signal path of the reset circuit;        (ii) 1/f noise in the channel of the first transistor of the electronic photo-charge detection circuit;        (iii) resistor (Johnson) noise in the channel of the first transistor of the electronic photo-charge detection circuit.        
From literature, it is known how to process photo-charge signals with the so-called correlated double sampling (CDS) technique, so that the first two noise sources (i), (ii) can be virtually eliminated, see for example A. J. P. Theuwissen, “Solid-state imaging with charge-coupled devices”, Kluwer, Dordrecht, 1995. CDS necessitates two measurements per photo-charge measurement: in a first step, the photo-charge detection circuit is reset to a certain, not precisely known voltage due to reset noise, and this voltage is measured. In a second step, the accumulated photo charge is transferred onto the same node of the photo-charge detection circuit, and the resulting voltage value is again measured. The difference of these two results yields the desired photo-charge value that is essentially devoid of the first two noise sources.
The third noise source (iii) turns out to be the key limiting factor for high-sensitivity photo-charge measurements: the input-referred photo-charge noise contribution of the first transistor in the photo-charge detection circuit is proportional to the effective capacitance at the gate of the first transistor, proportional to the square root of the temperature and proportional to the square root of the bandwidth. This relationship has been used in various ways to arrive at the state of the art in high-sensitivity photo sensing.
J. Janesick et al. describe in “Sub-electron noise charge coupled devices”, Proc. SPIE, Vol. 1242, pp. 238-251, 1990, how to exploit the square-root dependence of the photo-charge detection noise from the measurement bandwidth. They teach the principle of a charge-coupled device (CCD) with which the same photo-charge packet can be measured non-destructively and hence repeatedly. This so-called Skipper CCD produces a number n of statistically independent measurement values that are subsequently averaged. As predicted by theory, the photo-charge detection noise is reduced with the square root of the number n of averaged measurements, which corresponds to the measurement bandwidth. In this way, a statistical photo-charge detection uncertainty of less than one photoelectron has been obtained in practice. Unfortunately, the whole measurement process takes n times longer, so that the application of the Skipper CCD is essentially restricted to astronomy and a few other scientific fields.
A. Krymski et al. describe in “A 2e− Noise 1.3 Megapixel CMOS Sensor”, Proc. 2003 IEEE Workshop on CCDs and advanced image sensors” a method that exploits the parallelism of signal processing in image sensors that are fabricated with complementary-metal-oxide-semiconductor (CMOS) processes. They reduce the bandwidth in the signal path from the individual pixels to the output amplifier, and since this is done for several pixels simultaneously, the measurement process is not prolonged despite the effective bandwidth reduction of the photo-charge detection process. In addition, a CDS method is implemented to effectively cancel reset noise. In this way, a statistical photo-charge detection uncertainty of a few electrons is experimentally obtained at full video bandwidth of several MHz and at room temperature.
U.S. Patent Application Publication No. 2003/0042400 A1 (Hynecek, “Compact Image Sensor Layout with Charge Multiplying Register”) describes an alternate approach to high-sensitivity photo detection. A physical amplification mechanism, making use of the avalanche charge-multiplication effect, is employed. This does not reduce the noise contribution of the electronic charge-detection process, but since much larger charge signals are present at the input of the charge-detection circuit, the signal-to-noise ratio of the photo-charge measurement process is significantly improved. The observed statistical photo-charge uncertainty is close to one electron, at a full video bandwidth of several MHz and at room temperature.
Since the avalanche effect is implemented using a long series of CCD stages at rather high voltages of up to 20 V, this high-sensitivity photo sensor cannot be fabricated with industry-standard CMOS processes, and it cannot be operated with industry-standard 3.3 V supply voltages.